Batteries will lose power in storage regardless of location. The main difference is that batteries may lose power more quickly due to heat in a car, and the car has less space to stash spare batteries. Minimize battery leakage by keeping the batteries in the glove compartment or under insulation in the trunk, or in a Coleman-style cooler if you happen to just keep one in your car.
(see post #18 for the complete post)
Good battery tips, thanks!
One caution I would add is that the glovebox may still get too warm for batteries in the summer (depending on where you live, if the car is left in the sun, etc).